There have been numerous surface chemistry studies directed at increasing the surface energy of fluoropolymers to improve wettability and promote adhesion. Unlike most polymers, perfluoroalkanes are chemically inert. Accordingly, there are no direct chemical methods for the conversion of CF, CF2, and CF3 groups in perfluoroalkanes to any functionality. The most successful chemical reactions of fluoropolymers involve single electron reduction. However, this process most often changes the color of the treated film and the treated film does not have a significant shelf life. Plasma chemistry and surface grating have also been used to modify fluoropolymer surfaces. However, plasma chemical treatment is very expensive and results in a product with poor shelf life and migration of the functional groups to the bulk. Similarly, surface grafting is also very expensive because it requires high energy input to achieve the modification. Additionally, these methods are limited in their applicability to the treatment of films and not objects with more significant three-dimensional geometries. They also are not useful in treating porous objects or the interior of hollow objects. Another significant drawback to plasma chemical treatment is that it does not introduce discrete functional groups, but rather a mixture of species. Likewise, surface grafting usually results in inhomogeneous modification.
There are numerous reports from other research fields, however, of biopolymers spontaneously adsorbing to fluoropolymers. There is one report of a homopolymer (poly(L-lysine)) adsorbing to a fluoropolymer from aqueous solution. In this instance, it is the red action of interfacial free energy (the displacement of high energy water molecules from the fluoropolymer/water interface) that drives polymer or biopolymer adsorption (“hydrophobic interactions”). Poly(allylamine hydrochloride), polyethylenimine, and poly(acrylic acid) hash been adsorbed to poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) surfaces from aqueous solutions under varying conditions. Unfortunately, the extent of surface functionalization is minimal using these polymers.
What is needed is an easily used, economical, and effective surface treatment to modify hydrophobic solid-phase polymeric object, particularly those composed of fluoropolymers, and non-metal inorganic objects of any size and geometric conformation. The ideal method should improve surface wettability and promote adhesion, while maintaining desired performance characteristics of the modified object. The ideal treatment would result in a stable, durable product with a substantial shelf life and relatively uniform dispersal of discrete functional groups that do not migrate to the bulk.